For example, as disclosed in PTLs 1 and 2, the aforementioned power module includes a power module substrate obtained by bonding a metal plate to be a circuit layer onto one surface of an insulating substrate, and a power device (semiconductor element) mounted on the circuit layer. In addition, a heat sink such as a radiator plate or a cooler is provided on the other surface of the power module substrate to radiate heat from the power device (semiconductor element) in some cases. At this time, in order to alleviate the thermal stress caused by thermal expansion coefficients of the insulating substrate and the heat sink such as a radiator plate or a cooler, the power module substrate is configured such that a metal plate to be the metal layer is bonded onto the other surface of the insulating substrate and the metal layer and the aforementioned heat sink such as a radiator plate or a cooler are bonded to each other.
In the aforementioned power module, the circuit layer and the power device (semiconductor element) are bonded through the solder material.
Here, when the circuit layer is composed of aluminum or an aluminum alloy, for example, as disclosed in PTL 3, it is necessary to form a Ni plating film on the surface of the circuit layer by electroplating or the like and provide the solder material on the Ni plating film to bond the semiconductor element to the circuit layer.
In addition, when the circuit layer is composed of copper or a copper alloy, a Ni plating film is formed on the surface of the circuit layer and the solder material is provided on the Ni plating film to bond the semiconductor element to the circuit layer.